As is known, numerous MEMS devices are available today. For example, known to the art are the so-called MEMS reflectors, which are designed to receive an optical beam and vary the direction of propagation thereof, in a periodic or quasi-periodic way. For this purpose, MEMS reflectors include mobile elements formed by mirrors, the positions in space of which are controlled electronically.
In greater detail, in a generic MEMS reflector comprising a mirror, the control of position of the mirror is of particular importance for enabling scanning of a portion of space with an optical beam, which is made to impinge upon the mirror. In particular, the control of position of the mirror is a determining factor in the case of resonant MEMS reflectors, in which, in use, the mirror is made to oscillate in a substantially periodic way about a resting position, the period of oscillation being as close as possible to the resonance frequency of the mirror in order to maximize the angular distance covered by the mirror during each oscillation and hence maximize the dimension of the portion of space scanned.
For example, United States Patent Application Publication No. US2011/0109951 (incorporated by reference) describes a circuit for controlling the position of the mirror of a MEMS reflector of a resonant type, this mirror being arranged so as to turn, under the action of a motor of an electrostatic type, about an axis of rotation. In particular, the MEMS reflector comprises a fixed supporting body, made of semiconductor material, and a mirror, which is constrained to the fixed supporting body by means of a first spring and a second spring.
The fixed supporting body comprises a first stator subregion and a second stator subregion, which are connected, respectively, to a first stator electrode and a second stator electrode, and a first rotor subregion and a second rotor subregion, which are connected, respectively, to a first rotor electrode and a second rotor electrode. The first and second stator electrodes enable biasing, respectively, of the first and second stator subregions, whereas the first and second rotor electrodes enable biasing, respectively, of the first and second rotor subregions.
The mirror is mechanically arranged between the first and second springs, each of which has a respective end that is constrained to the fixed supporting body; in particular, the first and second springs are constrained, respectively, to the first and second rotor subregions. The mirror and the first and second springs hence form a resonant system, which has a respective mechanical resonance frequency. In general, the mechanical resonance frequency varies in time, for example, on account of temperature variations.
In greater detail, the voltages of the rotor electrodes and of the stator electrodes, and consequently the voltages of the rotor and stator subregions, are imposed in such a way as to cause oscillation of the mirror about the axis of rotation, with a mechanical oscillation frequency as close as possible to the mechanical resonance frequency. For this purpose, the first and second rotor electrodes are set at a biasing voltage (Vbias), whereas the first and second stator electrodes receive one and the same electrical command signal, formed by a high-voltage pulse train.
To cause oscillation of the mirror with a mechanical oscillation frequency as close as possible to the mechanical resonance frequency, it is necessary to know the mechanical resonance frequency and it is necessary to generate the pulses of the electrical command signal with appropriate frequency and phase, as a function of the position of the mirror. For these purposes, it is necessary to determine the position of the mirror, and in particular to determine the passages of the mirror through the resting position. Determination of the passages of the mirror through the resting position is made based on a signal proportional to a time derivative of a capacitance associated to the mirror, i.e., by means of a so-called “capacitive sensing”.
In detail, detection of the derivative of the capacitance associated to the mirror is performed during monitoring periods, in which the stator electrodes are set at ground, through an electronic circuit. Instead, during the so-called “driving periods”, the stator electrodes receive, once again through the electronic circuit, the electrical pulses and are hence set at a positive voltage such as to apply a torque to the mirror so as to keep the latter in oscillation. However, this positive voltage is so high as to cause saturation of an amplifier present inside the electronic circuit, this saturation making it impossible to detect the derivative of the aforementioned capacitance. The fact that the derivative of the aforementioned capacitance cannot be detected during the driving periods imposes a limitation on the duration of the driving periods, and hence of the pulses, which must thus be separated by the monitoring periods to enable updating of the estimate of the mechanical resonance frequency. Consequently, the pulses must be at a particularly high voltage in order to drive the mobile element properly. Moreover, given that the signal is proportional to the time derivative of the aforementioned capacitance, the determination of the corresponding position of the mirror requires to have available a processing unit, which must implement rather complex correlation algorithms.
U.S. Pat. No. 5,648,618 (incorporated by reference) describes, instead, a MEMS device in which, set above each spring, is a corresponding piezoresistor, the resistance of which varies as a function of the torsion that the respective spring undergoes. The MEMS reflector hence forms a piezoresistive sensor designed to generate an electrical signal indicating the angular position of the mirror, on the basis of which the position of the mirror is controlled. However, on account of the use of this piezoresistive sensor, the MEMS reflector is characterized by a certain circuit complexity; moreover, manufacture of the MEMS reflector must be made in such a way that, at the end thereof, the piezoresistors do not present residual stresses.
There is a need in the art to provide a MEMS device that will overcome at least in part the drawbacks of the known art.